As important factors of governing magnetic properties of electrical steel sheets, sizes and dispersing conditions of AlN and MnS precipitated in steels are taken up. This is why these precipitates themselves become to obstacles to movements of magnetic domain walls and deteriorate not only the magnetic flux density under a low magnetic field but also the iron loss, and in addition, they hinder grain growth during recrystallization annealing, and immature grain growth thereby of ferrite grains give bad influences to developments of recrystallization texture preferable to the magnetic properties.
It is known that coarser precipitates are preferable for the movements of the magnetic domain walls during magnetization. Based on such background, there has been disclosed prior art trying to provide the precipitations and coarsenings of AlN or MnS before the recrystallization annealing in the processes of making the electrical steel sheets. For example, Japanese Patent Laid-Open Specification 38814/74 checks di-solution of the coarse AlN during a slab soaking by lowering the soaking temperature thereof; Japanese Patent Laid-Open Specification 22,931/81 lowers amounts of S and O which precipitates as non-metallic inclusions; Japanese Patent Laid-Open Specification 8,409/80 controls formation of sulphides by addition of Ca or REM; Same 108,318/77, 41,219/79 and 123,825/83 coasen AlN by brief soaking in the slab before the hot rolling; and Same 76,422/79 utilizes selfannealing effect by coiling at super high temperature after hot rolling for coarsening AlN and accelerating growth of ferrite grain.
From a viewpoint of saving the energy in the process, it is advantageous to carry out a hot direct rolling from the continuous casting of a slab when performing the hot rolling. However, if depending upon this process, a problem occurs that the coarse precipitations of AlN and MnS are made insufficient, and for solving the problem, the slab is subjected to a brief soaking before the hot rolling.
However, although the soaking time is short, such a process which once transfers the slab into the heating and soaking furnaces, could not enjoy merits of saving energy brought about by the hot direct rolling, and further for providing precipitation of AlN, if the soaking time is short, the precipitation will be non-uniform at the inside and outside of the slab.